Principles of construction and functioning of digital radio communication networks of mobile and fixed radio services, digital broadcasting and television are described. The structure of radio systems for transmitting messages is considered, the properties of various radio channels and the principles of processing, transmitting and receiving signals in the channels of digital radio systems for transmitting messages, methods of increasing the efficiency of using the radio frequency spectrum allocated to radio systems through the use of multi-frequency and multi-level modulation, multiple access, etc.